As new materials are developed, it is advantageous to be able to weld or otherwise join the new material to itself or to already existing materials. Achieving satisfactory joint quality is an important milestone in a research and development scheme for any new material as satisfactory joint quality increases the likelihood that the new material can be used for widespread applications.
The production, separation and purification of hydrogen is an important industrial process, in part due to a continuing and increasing demand for hydrogen in electronic, fuel cell and chemical industries. Metallic membranes having high permselectivity, such as palladium (Pd) based membranes, are extensively used in the research and development of hydrogen production and separation. Even though Pd-silver (Ag) alloy foil or tube membranes are commercially available on a large scale, application of Pd—Ag membranes and foils encounter obstacles due to the lack of a satisfactory technique for joining the Pd—Ag alloy to normal metallic modules, such as to a stainless steel reactor or a separation cell.
Methods of joining thin metallic membranes with metallic modules, such as stainless steel, have been attempted in the past with varying degrees of success. Argon arc welding is generally unsatisfactory for welding thin Pd-based membranes with metallic modules because the high temperatures required for argon arc welding cause distortion and degrade or destroy the strength, ductility, and other metallurgical properties of the membrane. The high welding temperature can cause the membrane to be oxidized. The welded area can experience a hydrogen embrittlement problem. Hydrogen embrittlement is a process resulting in a decrease of the toughness or ductility of a metal due to the presence of atomic hydrogen.
Electronic beam welding is a more localized technique which produces little heat and can be used for more accurate micro-welding. However, when electronic beam welding is used to join a membrane with metallic module, micro-cracks in and near the welding area are often present that can cause leakage over time.
Brazing is another method that has been employed to join metallic membranes with metallic parts. A brazing filler of copper (Cu)—Ag has been used by us for joining Pd—Ag alloy membranes with stainless steel. The melting point of Cu—Ag alloy is high (>780° C.) and the Cu—Ag brazing filler can damage to the Pd—Ag alloy membrane.
U.S. Pat. No. 4,313,013 to Harris discloses attaching a tubular specimen of Pd—Ag alloy to stainless steel using a gold (Au) brazing filler. The melting point of Au is high (1064.43° C.) and can damage the Pd—Ag alloy.
Pd—Ag alloy foils are commercially available through cold-working. Thermal treatment of cold-worked alloys at high temperatures can cause significant boundary growth. High temperatures can also facilitate the concentration of impurities in the boundary area. Both boundary growth and concentration of impurities in the boundary area can lead to the formation of defects, which reduce the selectivity and longevity of the membrane. Accordingly, there exists a need to be able to use a lower temperature for joining.
When a brazing filler alloy having a lower melting point is used, some of the materials having a low melting point in the filler will vaporize and then deposit on the surface of the membrane. This contaminates the membrane during brazing or during subsequent use of the membrane at high temperatures in hydrogen.
U.S. Pat. No. 6,183,543 to Buxbuam discloses an apparatus for hydrogen purification. Buxbuam discloses that a leaking alloy tube membrane may be sealed by welding, soldering or brazing, and indicates that tubes can be sealed by high temperature cement or adhesive.
U.S. Pat. No. 6,458,189 to Edlund et al. discloses a membrane attached by contact adhesive to a screen. Flexible graphite gaskets can be used for connecting two or more membrane envelopes. Edlund et al. further disclose use of brazing, gasketing and welding as means for joining metal modules.
To obtain a gas-tight seal by graphite, a fitting head for tubular membrane sealing or a flange for planar membrane sealing is required. The fitting head or the flange occupy a large space compared to the size of the seal. Instability of the graphite in oxidizing or steam environments and difficulty making a gas-tight graphite make this method difficult to scale up.
Diffusion bonding techniques have been used in the aerospace industry to join similar or different members. It is often used in combination with superplastic forming for the fabrication of aircraft and aerospace components [see: D. V. Dunford and P. G. Portridge, Journal of Materials Science, 1987, 22, 1790-1798].
Zhang Guoge et al. in the Journal of Materials Science Letters 20, 2001, 1937-40 reported a diffusion bonding technique to bond Inconel alloy 718 with 17-4 PH stainless steel. Zhang Guoge et al. disclose a method of diffusion bonding using a constant temperature of 1000° C., which is too high for use with a Pd—Ag membrane.
U.S. Pat. No. 5,904,754 to Juda et al. discloses the diffusion bonding of copper at pressures of 1 atmosphere or less. Juda et al. disclose applying and controlling physical pressure by the torque load on four flange bolts. Furnace temperatures of between 200° C. and 350° C. are disclosed. Juda et al. disclose that copper badly deforms under gas pressure above about 200° C. Juda et al. teach that when carbon steel and stainless steel do not lend themselves to diffusion bonding unless they are coated with copper in which case the bonding takes place by copper migration.